Every sense except smell passes through the thalamus on its way up
Deep in the brain sit two egg-shaped lumps of grey matter, each about four centimetres long. Sight, hearing, touch and taste all route through them before reaching the cortex; only smell takes a detour. Once written off as a simple relay, the thalamus now looks more like a selective editor.
The name comes from the Greek word for chamber. The paired thalami sit near the centre of the brain between the cerebral cortex and the midbrain, forming part of the diencephalon, and each divides into at least 30 nuclei. A Y-shaped sheet of fibres splits each side into anterior, medial and lateral groups. Nearly all thalamic neurons send fibres to the cortex, and every cortical region examined so far sends fibres back. Swiss embryologist Wilhelm His Sr. identified it in 1893 as the main product of the embryonic diencephalon.
Each sensory channel has its own station. Signals from the retina reach the lateral geniculate nucleus and go on to the visual cortex at the back of the head; the medial geniculate nucleus handles sound; the ventral posterior nucleus forwards touch and body position. Strong feedback from the cortex into these relays is why researchers think the thalamus processes information as well as passing it on. Localised damage tends to produce specific sensory losses.
Its reach goes further. Loops between thalamus and cortex are thought to underpin consciousness, and the structure helps regulate sleep, wakefulness and arousal; damage can cause permanent coma. Its anterior nuclei link with the hippocampus in episodic and spatial memory. In a study of three monkeys, thalamic regions were involved in antisaccades, the skill of holding the eyes back from darting towards a stimulus.
Recent mouse work credits the mediodorsal thalamus with boosting just the cortical circuits suited to the current situation. Researchers reported that stepping up its activity cut the animals' error rate by more than 25 percent when choosing between conflicting cues. Blood reaches it through branches of the posterior cerebral artery, and a rare variant, the artery of Percheron, feeds both sides from one trunk.
Source: Thalamus
